1. Field of the Invention
The present invention relates generally to a color display device and, more particularly, to a color display device in which a simple optical system can be realized by separating diffracted light from illumination light.
2. Description of the Related Art
A light beam scanning apparatus is an apparatus for producing an image by causing a light beam to form spots on a photosensitive medium through scanning in an image forming apparatus such as a laser printer, a display device, a Light Emitting Diode (LED) printer, an electrophoto copying machine or a word processor.
As image forming apparatuses develop toward compactness, high speed and high resolution, light beam scanning apparatuses have been accordingly researched and developed to have the characteristics of compactness, high speed and high resolution.
Light beam scanning apparatuses for image forming apparatuses may be mainly classified into laser scanning type apparatuses using an f·θ lens, and image head printer type apparatuses, according to the light beam scanning method and the construction of a light beam scanning apparatus.
FIG. 1 is a perspective view showing the configuration of a conventional laser scanning apparatus using an f·θ lens.
Referring to FIG. 1, the conventional laser scanning apparatus includes a Laser Diode (LD) 10 that radiates a light beam in response to a video signal, a collimator lens 11 that converts the light beam, which is radiated from the LD 10, into parallel light, a cylinder lens 12 that converts the parallel light, which is passed through the collimator lens 11, into linear light coplanar with a scanning direction, a polygon mirror 13 that reflects the linear light, which is passed through the cylinder lens 12, while moving it at a constant linear velocity, a polygon mirror driving motor 14 that rotates the polygon mirror 13 at a constant velocity, an f·θ lens 15 that has a constant refractive index with respect to an optical axis, deflects the light, which is reflected by the polygon mirror 13 and which has a constant angular velocity, in a principal scanning direction, corrects aberrations of the light and focuses the corrected light on an illumination surface, a reflection mirror 16 that reflects the light beam, which is passed through the f·θ lens 15, in a predetermined direction and forms a dot-shaped image on the surface of a photosensitive drum 17, that is, an image plane, a horizontal sync mirror 18 that reflects the laser beam, which is passed through the f·θ lens 15, in a horizontal direction, and an optical sensor 19 that receives the laser beam, which is reflected by the horizontal sync mirror 18, and performs synchronization.
It is difficult for the above-described laser scanning type light beam scanning apparatus to achieve high-speed printing, due to the low switching speed of the laser diode 10 and the speed problem of the polygon mirror 13.
That is, to increase the scanning speed of the light beam, the polygon mirror 13 must be rotated using a high-speed motor. However, the high-speed motor is expensive, and the motor operating at high speed generates heat, vibration and noise, thus degrading operational reliability, so that a significant improvement in scanning speed cannot be expected.
Another scheme for improving the speed of the light beam scanning apparatus relates to an image head printing type light beam scanning apparatus using a multi-beam type beam formation apparatus.
Such a multi-beam optical scanning apparatus has a plurality of light emitting parts (laser heads) as light sources. The multi-beam optical scanning apparatus optically scans the surface of a recording medium using a plurality of light spots formed on the surface of the recording medium in such a way that a plurality of light beams radiated from the plurality of light emitting parts is focused by an imaging lens through an optical reflector.
In order to accomplish high-speed printing using only a single light spot, the number of times the surface of the recording medium is optically scanned per unit time must be significantly large. Meanwhile, the rotational speed of the optical reflector and the image clock cannot comply with the large number of optical scans. Accordingly, if the number of beam spots that simultaneously scan the surface of the recording medium increases, the rotational speed of the optical reflector and the image clock may be reduced in proportion to the number of beam spots.
In order to form a plurality of beam spots in the most effective manner, a laser element that functions as a light source has a plurality of light emitting points (light emitting parts) that can be independently operated.
Such a laser element having a plurality of light emitting points is commonly called a “monolithic multi-beam laser element.” When the monolithic multi-beam laser element is used, most of optical elements disposed behind the light source can be used for a plurality of light beams, so that the monolithic multi-beam laser element provides significant advantages in terms of cost, process and control.
FIG. 2 is a view illustrating a conventional laser scanning scheme in which laser scanning is performed by a plurality of beams produced by an LED array disposed in an image head.
Referring to FIG. 2, an LED array 21 is disposed in an image head 20 to have LEDs that can cover the width of printing paper, and generates a plurality of beams. Unlike the laser scanning scheme, printing can be performed on a line-at-a-time basis without using a polygon mirror or an f·θ lens, thus significantly improving printing speed.
This monolithic multi-beam laser element includes a so-called surface emitting laser (surface emitting type semiconductor laser).
The surface emitting laser emits light beams parallel to the thickness direction of a silicon layer, whereas a conventional semiconductor laser emits light beams perpendicular to the thickness direction of a silicon layer.
Furthermore, the surface emitting laser has the following characteristics. That is, the conventional semiconductor laser emits divergent light that has an elliptical cross section and considerably varied divergence angles, whereas the surface emitting laser can emit a circular beam that has a stabilized divergence angle.
However, the surface emitting laser has a problem in that an output light beam has an unstable polarization direction. Although the polarization direction can be controlled by the manufacturing method to some degree, it varies depending upon a light emitting point, ambient temperature and output.
The reflectance, transmittance and angle characteristics of the optical elements of an optical scanning apparatus, such as a polygonal mirror like an optical reflector, the scanning lens (f·θ lens) of an optical imaging system, and an echo mirror for changing an optical path, vary depending upon the polarization direction of an input light beam.
For this reason, when the monolithic multi-beam laser element including a surface emitting laser is used as the light source of an optical scanning apparatus, a plurality of beam spots that optically scans the surface of a recording medium have different intensities due to the different polarization directions of light emitting points. Further, the difference in intensity results in irregular pitch in an image, thus considerably degrading image quality.